I found a good solution to solve this issue. I leave it here if someone need it:
1.- First of all I create a folder named 'local' with the login file 'login.html'
login.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<body>
<center>Main Internet Feed is Down</center>
</body>
</html>
2.- Create a new Hotspot Server Profile copying the hotspot profile I use and change the html directory:
[root@DEMO] > /ip hotspot profile pr
Flags: * - default
0 * name="hsprof1" hotspot-address=1.1.1.1 dns-name="" html-directory=hotspot rate-limit="" http-proxy=0.0.0.0:0
smtp-server=0.0.0.0 login-by=cookie,http-pap,trial http-cookie-lifetime=3d split-user-domain=no trial-uptime=10m/4w2d
trial-user-profile=default use-radius=yes radius-accounting=yes radius-interim-update=received nas-port-type=ethernet
radius-default-domain="" radius-location-id="" radius-location-name="" radius-mac-format=XX:XX:XX:XX:XX:XX
1 name="hsprof_local" hotspot-address=1.1.1.1 dns-name="" html-directory=local rate-limit="" http-proxy=0.0.0.0:0
smtp-server=0.0.0.0 login-by=cookie,http-pap,trial http-cookie-lifetime=3d split-user-domain=no trial-uptime=10m/4w2d
trial-user-profile=default use-radius=yes radius-accounting=yes radius-interim-update=received nas-port-type=ethernet
radius-default-domain="" radius-location-id="" radius-location-name="" radius-mac-format=XX:XX:XX:XX:XX:XX
3.- Create a netwatch to some server [I use google dns 8.8.8.8 ] If is down I change the hotspot profile to the local one and remove all active users. If is up I turn it back the the normal hotspot profile:
/tool netwatch
add comment="DNS Google - Comprobacion conectividad" disabled=no down-script="\
:local dumplist [/ip hotspot active find]\r\
\n:foreach i in=\$dumplist do={\r\
\n\t/ip hotspot active remove \$i\r\
\n}\r\
\n/ip hotspot set hotspot1 profile=hsprof_local\r\
\n" host=8.8.8.8 interval=1m timeout=1s up-script=\
"/ip hotspot set hotspot1 profile=hsprof1\r\
\n"
That's it!!
It works grate